The reaction calorimeter is commonly employed to study several aspects of a desired chemical reaction like heat of reaction, heat release rate, accumulation, maximum temperture of synthesis reaction (MTSR) etc. In order to assess the thermal risks associated with a chemical reaction, both the desired and undesired (i.e., decomposition) reactions must be studied. In this paper we report in detail the practical procedure followed for the assessment of thermal risk of a chemical reaction and thereby identification of Stoessel’s criticality class. The kinetics and energy potential of the undesired decomposition reaction are studied using differential scanning calorimeter (DSC). The decomposition onset (Tonset) obtained from DSC was used in Arrhenius kinetic model (zero order assumption) to estimate TD24. Thus obtained TD24 value is on the conservative side and can be used for safety purposes. We also discussed how this thermal assessment further led to the chemical process improvisation in terms of thermal safety for a safe and efficient scale up. We have discussed an example of bromination reaction where azobisisobutyronitrile (AIBN) decomposes to form a free radical and initiates the chain reaction. The reaction is found to be associated with high thermal risks corresponding to a Stoessel’s criticality class 5. Process was optimized by varying the feed profile of the AIBN in solvent solution to reduce the Stoessel’s criticality from class 5 to class 2. We have also discussed the synthesis of tetra phosphate ester where the chemical process was improvised in terms of safety by classifying the reagent isopropyl addition into two zones; one critical zone of addition (control measures are required) and the other non critical zone.